1. Field of the Invention
The present invention relates to refrigerated vending machines and, more particularly, to power-conserving refrigerated vending machines. A major objective of the present invention is to provide continuous availability of cold product with temporarily reduced power consumption.
2. State of the Art
Vending machines provide for cost-effective delivery of consumer items. In principle, they provide full-time product availability with minimal intervention by a human operator. However, full-time operation can result in wasted energy consumption as the machine may be on for long intervals of inactivity. The concern for energy consumption is especially acute in the case of refrigerated vending machines.
Refrigerated vending machines maintain their contents below ambient temperatures. There can be various reasons for keeping the dispensable items cold. Cold generally helps preserve perishable food items. In some cases, for example, with soda and other beverages, the items may taste better chilled. In other cases, the refrigerated vending machine can be used in conjunction with a heating device, such as a microwave oven, to allow chilled food, e.g., such as sandwiches, to be heated to a desirable temperature before consumption.
Typically, the dispensable items are maintained within a chamber that is thermally insulated from the exterior of the vending machine. A cooling system withdraws heat from the chamber. The cooling system can include an evaporator, a compressor, a condenser, and a metering (flow constricting) device.
When the cooling system is on, coolant liquid, e.g., Freon, enters the evaporator. The evaporator is thermally coupled to the refrigerated chamber. The coolant liquid is generally colder than the chamber so that the coolant removes heat from the chamber. The liquid evaporates as it absorbs the heat.
The evaporated coolant is pumped out of the evaporator through a suction line by a compressor. The compressor increases the pressure of the coolant, raising its temperature in the process. The pressurized coolant is then directed to a condenser via a discharge line.
The condenser couples the coolant to a chilled environment. This causes the coolant to give up heat and condense into a liquid. The liquid flows through a liquid line, including the flow meter (which is basically a flow restriction) back to the evaporator to begin another cooling cycle.
The evaporator removes heat from the nearby chamber air. To ensure that the cool air reaches the dispensable items and to ensure a uniform temperature within the chamber, the chamber air is circulated. Generally, one or more fans are operated within the chamber interior to effect this circulation.
One or more thermo-sensors monitor the temperature inside the chamber. Typically, there is a desired temperature range for the vended items, for example, 0°-2° C. (˜32°-36° F.) for sodas. When the chamber temperature reaches the higher threshold, the compressor is activated and the cooling process begins. When the chamber temperature falls to the lower threshold, the compressor is turned off, and cooling effectively halts. Another cooling cycle can begin when the temperature reaches the upper threshold due to inevitable heat transfer through the chamber wall.
Refrigerated vending machines consume considerable electric power. Typically, most of the power consumed by a refrigerated vending machine is consumed by the cooling system, and especially by the compressor, even though it is not operated continuously. However, the fans, the dispensing mechanism, the money handling mechanisms, panel lights, sensors, and control electronics all consume power. For reasons of energy conservation and cost, it is desirable to be able to reduce the energy consumed by a refrigerated vending machine without adversely affecting its service (to patrons) and its economic (to the vending machine owner) aspects.
The most straightforward approach to saving energy is to disconnect AC power. For example, a vending machine could be turned off during non-business hours, e.g., from 10 pm to 6 am. To avoid the inconvenience of manual activation and inactivation, an external timer can be used to control AC power to the vending machine. However, whether power to the vending machine is switched by a human operator or a timer, potential patrons are denied dispensable items during off hours.
Parent patent application Ser. No. 09/182,586 discloses an external power control system for a vending machine that includes an occupancy sensor. This can be used to ensure a vending machine is on whenever people are in its vicinity. An ambient thermo-sensor can also be included to determine a reactivation time to prevent the dispensable items from become unacceptably warm.
Using even an effective external device is not ideal. From a manufacturing viewpoint, there is duplication of components. For example, the external power controller must have-its own housing, its own power supply, and own control electronics. Also, the vending machine operator must manage two devices instead of one. From a power-conservation standpoint, power is less likely to be saved if it requires a separate device to be purchased, installed, and set up. Accordingly, refrigerated vending machines with built-in power-conservation features are desired.
On approach allows a vending machine to be programmed to allow different target cooling set points at different times in this case, for example, a vending machine can be programmed to have a higher set point during periods of expected inactivity (e.g., non-business hours). Raising the set point can have a significant impact on power consumption since power is consumed roughly in proportion to the differential between the ambient temperature and the desired chamber temperature. When the set point is higher, the compressor duty cycle is reduced, thus reducing overall power consumption.
This varying set-point approach is attractive in that the vending machine is always on and ready to do business. The dispensable items are warmer than ideal, but only by an amount determined by variations in the set point. However, there is generally not much latitude for raising set points.
The set points used in normal operation are typically chosen carefully to achieve optimal cooling of dispensable items without wasting power. Power savings beyond those achieved during normal operation typically exact a penalty, e.g., possibly compromising the freshness or taste of the dispensable items. What is needed is a system that provides for power conservation without adverse affects on the dispensable items.